[alpal]

Embarrassing problem as I should have noticed this before.


I am putting my 10" f4 Newt back into service after 6 years and this is where I'm up to:
I hadn't cleaned the secondary mirror yet because I wanted to check it for offset -
just in case it might have a small error in it.
When I used my digital vernier – after about an hour of making difficult measurements -
I came up with a result that I thought must be wrong –
It tells me that the secondary mirror is offset in the wrong direction by 9.4 mm.
An online calculator says the offset should be 5.5 mm towards the primary mirror –

offset is approximated by: M/(4*F) where M is the minor axis of the secondary mirror and F is the F ratio.
So for my 10 inch f/4 with secondary minor axis diameter of 88mm, the offset will be 88/(4*4) = 5.5mm.

but it’s offset the other way by 9.4 mm towards the front opening of the telescope!
Then I looked in the focuser and sure enough I can see that it’s wrong
without even measuring it. See pic attached.
( I placed a red shirt on the opposite side of the tube to make a photo more obvious
and I focused on the edge of the secondary mirror using my DSLR on a tripod. )

I don’t know how this could have happened.
It was bought from TS in Germany in 2015 and they set up the position of the secondary mirror by
installing the spider and focuser.
How could professionals who charge a lot of money and make 100s of telescopes make such an error?
How did my telescope even work at all like that?
There is also the question of the other offset which would
be away from the focuser by the same amount 5.5mm.
The center of the spider is centralised so it may not have any offset
away from the focuser.
I'm not sure how to measure that.
Any comments?


cheers
Allan